1. Field of the Invention
The present invention generally relates to the field of electronic circuits and, more specifically, to electronic circuits comprising a detector of radio-frequency signals, for example, for demodulation purposes.
2. Discussion of the Related Art
Several techniques for demodulating an amplitude-modulated radio-frequency signal are known.
A first technique uses a diode with an anode which receives the signal to be demodulated and with a cathode which is connected to the input of a low-pass filter providing the envelope signal. This type of envelope detector requires that the signal to be demodulated reaches the diode with an amplitude greater than the diode threshold (several hundreds of millivolts).
To detect signals modulated with a lower amplitude, an amplifier is interposed upstream and/or downstream of the diode. A disadvantage then is the increased power consumption of the amplifier. This power consumption increases along with the carrier frequency, and thus with the possible transmission rate.
Another so-called coherent demodulation technique uses a differential amplifier receiving the signal to be demodulated. The amplifier is followed by a multiplier (differential outputs of the amplifier) having its output driving a low-pass filter. A disadvantage here again is the circuit power consumption.
Another so-called super-regenerative technique uses an oscillator controlled by the output of a differential amplifier receiving the signal to be demodulated and having its output rectified by a diode, upstream of a low-pass filter. Here again, a problem is posed with the high required power consumption.
An increase in the transmission rate generates an increase in the consumed power. Such an increase is not desirable for consumption reasons Such is, for example, the case for electromagnetic transponders equipped with an on board power source (battery).
In other cases, the power supply may not transmit the desired rates. Such is, for example, the case for electromagnetic transponders remotely supplied by a high-frequency field radiated by a read or read/write terminal. The power available on the transponder side decreases as the transponder is further from the reader or from the terminal which generates the high-frequency field.
In other cases still, it may be desired to detect that a transponder (having its own supply source) is present in the electromagnetic field of a read/write terminal. This amounts to detecting, on the transponder side, the presence of the field. The sooner a field is desired to be detected when a transponder comes closer to a reader, the lower the level to be detected.
EP-A-0 982 847 discloses a high frequency power detection circuit comprising a field effect transistor to the gate of which the high frequency signal is input, and a load capacitor and a load resistor connected in parallel between the source of the field effect transistor and the ground.